When attempting to copy an item, Sharegate displays the following warning:
There are multiple destination items that correspond to the given key values for the lookup item '1'. The first one was assigned, but it might not be the one that was expected.
The warning occurs when the given field points to an ID column and Sharegate finds multiple corresponding items at the destination. Since the ID is not preserved at the destination, Sharegate needs to map the source item to a destination item based on the Unique Key properties. These properties are usually enough to ensure only one match at the destination, but it is possible that they are not sufficient.
The warning indicates that of the multiple matching items at the destination, the first one was assigned. This tells the user to verify if the item that was assigned at the destination is the correct one in the Sharepoint UI.